Servo-control system

ABSTRACT

A servo-control system comprising a bridge circuit, a detector circuit, a motor, and a control circuit which functions as a pulsive switching circuit, said detector circuit being controlled by the output of said bridge circuit, said motor being controlled by the output of said bridge circuit through a said detector circuit under the control of said control circuit and controlling and bringing said bridge circuit to a balanced condition.

I United States Patent 1191 1111 3,743,912

Mashimo July 3, 1973 1 SERVO-CONTROL SYSTEM [56] Relerences Cited [75] Inventor: Yukio Mashlmo, Tokyo, Japan UNITED STATES ATENTS Assigm Kahushiki K951191990 3:113:22 115132; $123,212,312;;;1;"::"""':::::: 3135221? Japan 3,327,186 6/1967 Gregory et a1 318/663 x 22 il Oct 1 1970 3,427,941 2/1969 Metzger 318/599 X 3,426,662 2/1969 Sevin 318/599 X [21] App]. N0.: 81,680 3,450,969 6/1969 Sato et a1 318/681 X 3,472,142 10/1969 Fahlenberg 318/663 3,652,912 3 1972 B d 318 99 [30] Foreign Application Priority Data or onam I5 06:. 28, 1969 Japan 44/86250 a Dobeck Apr. 3, 1970 Japan 45/28465 M G| & Toren Apr. 20, 1970 Japan 45/33615 June 19, 1970 Japan 45/53661 57 ABSTRACT g japan A servo-control system comprising a bridge circuit, a J c 970 Japan 45/8452 detector circuit, a motor, and a control circuit which 1970 "gs/39195 functions as a pulsive switching circuit, said detector 1 1970 Japan 45/45826 circuit being controlled by the output of said bridge cir- J ay 1970 z 45/614ol cuit, said motor being controlled by the output of said 1970 2: 45/64316 bridge circuit through a said detector circuit under the une f p control of said control circuit and controlling and 52 u.s. c1 318/599, 318/640, 318/663 bmgmg br'dge a [51] Int. Cl. G061) 11/28 31 Claims, 30 Drawing Figures [58] Field of Search 318/663, 640, 599

r E 1 M D c A c Patented July 3, 1973 3,743,912

15 Sheets-Sheet 1 mvsmon VUKIO MASHIMO BY 95w) 475102 N awo fzmEvs Patented July 3, 1973 3,743,912

15 Sheets-Sheet 2 INVENTOR. vumo MASHIMO BY mmgaw amdTemz ATTORNEYS Patented July 3, 19-73 3,74 ,912

15 Sheets-Sheet 3 Fl G. 5

F y l l J Z 2 u. 1 I

Patented July 3, 1973 3,1 3,912

15 Sheets-Sheet 4 J J J .1

3c F F INVENTOR. ll/RIO MASHIMO l/flc way/7475mm ATTORNEYS Patented July 3,1913 3,743,912

15 Sheets-Sheet 5 I NVENTOR.

YURI-O MASH/M0 m ammo 751m ATTORNEYS Patented July 3, 1973 I 3,743,912

15 Sheets-Sheet 7 INVENTOR. YUKIO MASHIMO WamO W m6 ATTORNB'S Patented Jul 3, 1973 3,743,912

15 Sheets-Sheet 8 Driving 1 I Time INVENTOR.

VUKIO MASHIMO BY ardmd 761w? fiTTOflN Patented July 3, 1973 3,743,912

15 Sheets-Sheet 9 INVENTOR. YURIO MASHIMO m 40 0/170 Znm fiTTORNEYS Patented July 3, 1973 3,143912 15 Sheets-Sheet 10 l NVEN TOR.

YUHIO MASHIMO BYmWWEpq n ATTORNEYS Patented July 3, 1973 15 Sheets-Sheet 11 INVENTOR. VUHIO HASHIMO m WTORNEYS Patented July 3, 1973 3,743,912

15 Sheets-Sheet 12 FIG.23

INVENTOR YUM/0 MASHIMO We wamoEmz ATTDRNEYS Patented July 3, 1973 i 3,743,912

15 Sheets-Sheet 15 INVENTOR. YUR/O MASH 1N0 BY w emu 157m? fiTTO R N EVS Patented July 3, 1973 3,743,912

l5 SheetsSheet 14 INVENTOR. VUH/O MASHIMO BY yl wa no E'lm (mt ATTORNEYS SERVO-CONTROL SYSTEM The present invention relates to a servo-control system for a small sized portable machine, such as an automatic iris control device of a movie camera or the like.

In general, a servo-control system for driving an automatic iris control device of a movie camera or the like, has such a defect as a hunting phenomenon and an unstable operation caused by an inertia of a motor or the like. In order to prevent the defect, there has been employed a motor with a damping coil, to which current of an opposite polarity is given a tachogenerator with feedback to its input, or the like. However, all of these make thesystem complicated and undesirable for a servo-control system of a small-sized portable machine and have such a defect such as insufficiency in braking operation due to a tendency of a non-sensitive zone being enlarged.

The present invention is to provide an improved servo-control system, which has overcome the above defects of the conventional devices.

One of the objects of the present invention is to provide a servo-control system comprising a bridge circuit, a detector circuit, a motor, a control circuit which functions as a pulsive switching circuit, and a start switch for activating the control circuit under the control of the output of said detector circuit, said detector circuit being controlled by the output of said bridge circuit, said motor being controlled by the output of said bridge circuit through said detector circuit under the control of said control circuit and controlling and bringing said bridge circuit to a balanced condition.

Another object of the present invention is to provide a servo-control systemcomprising a comparator circuit, a detector circuit, a motor and a chopper which functions as a pulsive switching circuit, said chopper chopping at a certain time interval for the brake of the system, said detector circuit being controlled by the output of said comparator, said motor being controlled by the output of said comparator circuit through said detector circuit under the control of said chopper, and controlling and bringing said comparator circuit to a balanced condition.

Still another object of the present invention is to provide a servo-control system comprising a comparator a detector circuit, a motor, a control circuit which functions as a switching circuit, said detector circuit being controlled by the output of said comparator, said motor being controlled by the output of said comparator through said detector circuit under the control of said control circuit and controlling and bringing said comparator to a balanced condition, and said control circuit comprising an integrator and a feed-back connection from said integrator to the detector circuit, said integrator being controlled by the current through the motor.

Features of one embodiment of the present invention lie in a servo-control system, comprising a motor, having two driving coils, wound with a polarity opposite to each other, and being driven by a two-output astable multivibrator. By controlling the pulse width of the outdetected by a detector circuit and the astable multivibrator is controlled by the output from the detector circuit, and the output of the multivibrator drives the motor, having two driving coils, which are wound with a polarity opposite to each other.

Features of a third embodiment of the present invention lie in a servo-control system, comprising a motor having a single driving coil driven by a two-output astable multivibrator. By controlling the pulse width of the output of the multivibrator with an unbalanced output of the bridge, the motor is normally or reversely rotated to operate an automatic iris control mechanism or the like.

Features of a fourth embodiment of the present invention lie in a servo-control system, in which an effective braking power can be obtained by an intermittent derive of the motor through an electric or electromagnetic chopper.

In a fifth embodiment of the present invention, the motor is driven by a pulse current from a pulse oscillator and the driving pulsive power of the motor is decreased before the stop of the motor by the change of the pulse period, and thus, a braking action can be electronically performed. Naniely, the pulse period of the pulse oscillator for driving the motor changes correspondingly with the unbalanced output from the bridge.

In a sixth embodiment of the present invention there is provided a braking circuit of a servo-control system for operating an automatic iris control device of a movie camera or the like, characterized by that a braking action is performed by an electronic chopper circuit, without particularly using a damping coil or the like, and the non-sensitive zone is not enlarged and the servo-motor can be started in a quite short rising time.

In a seventh embodiment of the present invention the motor is driven under the control of a pulse signal from .a pulse oscillating circuit, which comprises a time constant circuit and a thyrister with four terminals. The braking action can be electronically performed in the servo-control system, and the circuit construction may be simplified by employing the thyrister of a four terminal construction. a

In an eighth embodiment of the present invention a current, passing through a driving coil of the motor is given with an intermittent wave form to get a braking effect, and thus, the instability of the operation caused by a hunting, is eliminated to perform a stable operation. That is, the voltage drop in the driving coil is utilized to control the switching circuit through integration of the current in response to the voltage drop and to cause interruption of the driving current of the motor.

In a ninth embodiment of the present invention the motor is rotated by the output of the astable multivibrator, which is started or stopped by the signal from a detector circuit.

In a 10th embodiment of the present invention the motor is caused to rotate in a direction in response to the output from the detector circuit, by a switch under the control of a pulse oscillator and the motor is caused to terminate to function as a generator to effect a brak- Thus, the present invention has many advantages that the circuit can be simplified, and it is easy to regulate the length of the intermittent wave pulse and moreover, it is easy to adopt integrating circuitries for miniaturizing the system.

Further, the present invention has an advantage that power consumption is remarkably reduced by a switching control of the system and the system constituting elements.

The present invention shall be described in reference to the attached drawings in which:

FIG. I is a block diagram of a servo-control system in accordance with the present invention; FIG. 2 is a first embodiment of a servo-control system for a movie camera according to the present invention; FIG. 3 shows a wave form in a servo-control system in accordance with the present invention; and FIG. 4 illustrates the operational characteristics of the system shown in FIG. 2.

FIG. 5 is a second embodiment of a servo-control system in accordance with the present invention; and FIG. 6 shows a wave form in the system shown in FIG. 5.

FIG. 7 is a third embodiment of a servo-control system in accordance with the present invention; and FIG. 8 shows a wave form in the servo-control system shown in FIG. 7.

FIG. 9 is a block diagram of a fourth embodiment of a servo-control system in accordance with the present invention; FIGS. 10 to 13 are schematic circuit diagrams, showing examples of the fourth embodiment in accordance with the present invention.

FIG. I4 is a schematic circuit diagram showing a fifth embodiment of a servo-control system in part in accordance with the present invention; FIG. 15 shows a wave form for illustration of the transient characteristics of the servo-control system shown in FIG. I4; and FIG. 16 is a schematic circuit diagram showing the fifth embodiment of the servo-control system in whole.

FIG. 17 is a schematic circuit diagram, showing a sixth embodiment of a servo-control system in accordance with the present invention; and FIG. 18 is a schematic circuit diagram, showing a modification of the sixth embodiment.

FIG. 19 is aschematic circuit diagram,-showing a seventh embodiment of a servo-control system in accordance with the present invention; FIG. 20 shows a wave form of the driving current of a motor; and FIG. 21 is a schematic circuit diagram, showing a modification of the seventh embodiment.

FIG. 22 is a block diagram of an eighth embodiment of the present invention; FIG. 23 shows a wave form in the embodiment shown in FIG. 22; and FIG. 24 is a schematic circuit diagram of the eighth embodiment.

FIG. 25 is a schematic circuit diagram, showing a nineth embodiment of a servo-control system in accordance with the present invention; and FIGS. 26 to 28 are schematic circuit diagrams of modifications of the ninth embodiment in part.

FIG. 29 is a schematic circuit diagram showing a lOth embodiment of a servo-control system according to the present invention.

FIG. 30 is a schematic circuit diagram, showing an llth embodiment of a servo-control system in accordance with the present invention.

The same numbers and designations throughout the attached drawings show the same elements with similar functions.

In FIG. I, B is a bridge circuit possibly including a differential amplifier circuit, D is a detector circuit, C is a control circuit, M is a motor and Sc is a start switch for the controller circuit. Fs shows a feed-back loop of the system.

When the system is actuvated, the unbalanced output from the bridge circuit B starts the control circuit C with the aid of the start switch Sc through the detector circuit D, and the output from the control circuit C drives the motor M to bring the bridge circuit B into a balanced state through the loop Fs.

The first embodiment shown in FIG. 2 will be explained referring to FIGS. 3 and 4. l-a l-a and I-a show respectively a wave from in a coil 18, and l-b I-b and 1-b a wave form in a coil 19. 1 is an iris control mechanism, interlocking with a motor M and 2 is a photo-conductive element (for example of CdS), the incident light to which being regulated by automatically controlling the iris. 3 is a variable resistor for setting a photographing information, such as the film sensitivity, etc. 4 and 5 are resistors, which compose a bridge circuit with the resistors 2 and 3. 6 and 7 are first-stage transistors in a bridge output detector circuit; 8 and 9 are amplifier transistors; 10 and 11 are output-stage transistors of the detector circuit. 12 and 13 are transistors, composing an astable multivibrator. l4 and 15 are resistors and 16 and 17 are capacitors. These compose a pair of time constant circuits. 18 and 19 are driving coils, wound with a polarity opposite to each other to compose the motor M 20 is a starter transistor for controlling actuvation and actuation of a multivibrator, 21 is a main switchfor the system, and 22 is a battery as a power source.

Next, the operation of the embodiment shown in FIG. 2 will be explained. If the incident light to the photo-conductive element 2 reaches a suitable level and the resistors 2, 3, 4 and 5 are balanced, all of the transistors 6, 7, 8, 9, I0 and 11 in the detector circuit become non-conductive and as the based the starter transistor 20 is connected with the output terminals of the detector circuit, the transistor 20 becomes nonconductive. Therefore the astable multivibrator circuit is not actuated andno current flows through the two driving coils 18 and 19 of the motor M Thus the motor is maintained in a stop condition and the bridge circuit remains in a balanced condition. If the incident light to the photo-conductive element 2 decreases and its resistance value is increased, the first-stage transistor 7 becomes conductive, and then, the transistors 9 and Ill become conductive, whereby the starter transistor 20 becomes conductive and thus the multivibrator begins to actuate. At this time, the resistor 15 of one of the time constant circuits of the multivibrator is connected in parallel with the output circuit of the output transistor 11 of the detector circuit. Thus the time constant of this circuit becomes shorter and such pulses as shown by 1-11 and l-b in FIG. 3 passes through the driving coils 18 and 19 respectively. The motor M is rotated in response to the difference between the widths of both pulses, and the iris control mechanism is set in rotation to bring the bridge circuit back to a balanced condition.

FIG. 4 shows the relation between the driving pulsive torque of the motor and the output from the bridge circuit. a shows the driving torque while B shows the braking torque. In a condition shown in the left-half side of the drawing, the motor M, is supposed to rotate normally as a positive torque being generated as a difference between the positive torque and the negative torque. While in a condition shown in the right-half side of the drawing, the motor M is reversely rotated as a negative torque being generated.

When the bridge circuit approaches to a belanced condition, pulses of such wave forms as shown by 1-a,, and l-b, in FIG. 3, pass through two driving coils 18 and 19. When the difference of the pulse widths become zero, the motor is stopped. In this condition, the bridge circuit is balanced and the transistors 7, 9 and 11 become non-conductive. Thus, the starter transistor 20 becomes non-conductive and the actuation of the multivibrator is stopped.

Next, when the incident light to the photoconductive element 2 increases the bridge circuit becomes unbalanced again, and in this case the transistors 6, 8 and become conductive and then, the transistor becomes conductive. Thus the multivibrator begins to actuate and such pulses as shown by l-a and l-b in FIG. 3 pass through the motor M Accordingly, the motor M is set in motion in response to the difference of the pulse lengths. The bridge circuit is brought back to a balanced condition again. In this case, the resistor 14 of the multivibrator is put in parallel with the impedance of the output transistor 10.

As above, in the first embodiment of the present invention, the servo-control system is assured of a quite high degree of precision as clearly understood from FIG. 4.

The second embodiment shown in FIG. 5 will be explained referring to FIG. 6. 2-a 2-a and 2-a show wave forms in a driving coil 18, and 2-b 2-b and 2-b show wave forms in a driving coil 19. 6 and 7 are transistors of output detector circuits 23 and 24.

Next, the operation of the embodiment shown in FIG. 5 will be explained. When a pair of time constants in the multivibrator circuit become equal the bridge circuit is supposed to be balanced and the motor is stopped. For this the values R and R of the resistors 14 and 15 are selected to be R R in the circuit, if supposed C C in which C is the capacitance of a capacitor 16 and C is that of a capacitor 17. In the supposed condition, if there is a small amount of light incident from the body to be photographed and the resistance value R of the photo-conductive element 2 is larger than the preset value R of the variable resistor for setting the photographing information, the transistors 23 and 24 of the bridge output detector circuit become non-conductive and by the above condition of R R the outputs of the multivibrator give, two driving coils 14 and 15, a pair of pulse currents as shown by 2-a and Z-b, in FIG. 6. Thus, the motor is normally rotated in response to the difference of the pulse length of 2- a and 2-12 and the iris control mechanism 1 is rotated by the motor M to regulate the amount of the incident light to the photo-conductive element 2 so as to make R equal to R If R R the transistors 23 and 24 become conductive and the output circuit of the output transistor 24 is put in parallel with the resistors 14 of the multivibrator. Thus the equivalent resistance R of the time constant circuit with the resistor 14 is decreased and the resistance R becomes equal to R In this condition, the output of the multivibrator has such pulses as shown by 2-a and 2-b, in FIG. 6, and the driving torque is balanced with the braking torque in the motor, which responds to a stop condition.

When the amount of the incident light is remarkably increased, the resistance R, becomes much smaller than R and the transistors 23 and 24 become conductive again, whereby the output circuit of the transistor 24 is put in parallel with the resistor 14 again, and the equivalent resistance R becomes smaller than R In this condition pulses as shown by 2-a and 2-b are given from the multivibrator to the motor M to cause a rotation motion in a reverse direction in response to the difference of the pulse lengths until a balanced condition of R R obtained.

In this embodiment, the detector circuit is of one channel type and the output transistor 24 functions as a valuable impedance element. And thus the detector circuit can be simply constructed with a sufficient braking function. So, the effective system for prevention of a hunting can be constructed in a small size to be useful as a servo-control system for a movie camera or the like.

The third embodiment shown in- FIG. 7 will be explained referring to FIGS. 4 and 8. In this embodiment 9 is a transistor for reversing phases; 8 and 25 are output transistors; and 12, 13, 26 and 27 are transistors composing an astable multivibrator of a bridge connection structure. 14 and 15 are resistors, and 16 and 17 are capacitors. 28 and 29 are diodes for preventing a reverse current. M is a motor.

Next, the operation of the third embodiment will be explained. If the amount of light incident to a photoconductive element 2 reaches a suitable value for a balanced condition of the resistance bridge circuit, all of the transistors of the output detector circuit become non-conductive and the starter transistor 20, connected with the output terminals of the detector circuit, becomes non-conductive. Thus, the astable multivibrator circuit does not begin to oscillate and no current is given to a single driving coil of the servo motor M Thus the motor is kept in a stop condition and the bridge circuit remains in a balanced condition. If the amount of light incident to the photo-conductive element 2 is decreased and the resistance value becomes larger, one of the first stage transistor 7 becomes conductive due to the output from the resistance bridge circuit and the transistors 9 and 25 become conductive. Then, the starter transistor 20 becomes conductive and the multivibrator begins to oscillate. At this time, the resistor 14'in one of the time constant circuits of the multivibrator is put in parallel with the impedance of the output transistor 25 of the detector circuit. The time constant becomes shorter and a positive or negative pulse, having different lengths, as shown by 3-c in FIG. 8 passes through the single driving coil of the motor M The motor M is rotated in a normal direction due to the difference of the length of the positive pulse and the negative pulse, to operate the automatic iris control mechanism and to bring back the bridge circuit to a balanced condition.

In this case, the relation of the driving torque of the motor to the output from the bridge circuit generated,

by the motor M, with a common coiling is similar to that shown in FIG. 4.

In the condition of the left-half side of FIG. 4, the motor is supposed to be normally rotated, as a positive resultant torque being generated, and in the condition of the right-half side, it is reversely rotated, as a negative resultant torque being generated.

When the bridge circuit approaches to a balanced condition, pulses having such a wave form as shown by 3-0 in FIG. 8 passes through the common driving coiling, and when the difference of the length between the positive pulse and the negative pulse becomes zero, the motor stops. In this condition, the bridge circuit is balanced and the transistors 7, 9 and 25 turn off. So, the starter transistor also turns off and the oscillation of the multivibrator is stopped.

Next, when the amount of light incident to the photoconductive element 2 is increased, the transistors ti and 8 become conductive. Then, the starter transistor 20 becomes conductive and the multivibrator begins to oscillate. Such pulses as shown by 3-0 in FIG. 3, passes through the common coiling of the motor M Accordingly, the motor M is driven in a reverse direction due to the difference of the length between the positive pulse and the negative pulse, and the bridge circuit is brought back to a balanced condition again. In this case, the resistor 15 of the multivibrator is put in parallel with the impedance of the output transistor 8.

In the above embodiment, the motor lVil having a single driving coiling is driven by two outputs of the astable multivibrator composed in a bridge structure. This embodiment is prevented from a hunting phenomenon because the braking torque is always given as well as the driving torque. And when the bridge circuit is balanced, the multivibrator is stopped in oscillation. So, a stable operation can be performed and the servocontrol system can be constructed with a quite high degree of precision.

In the fourth embodiment shown in FIG. 9, B is a comparator circuit such as a bridge circuit, a differential amplifier circuit, which comprises a variable resistor, interlocking with a motor M; D is a detectorcircuit; C is a controller circuit such as achopper circuit or a pulse oscillating circuit. The controller circuit C may be selectively connected at a position as illustratively shown in FIG. 9. A is an amplifier circuit and E is a power source. The unbalanced output from the bridge circuit 13 is put in the detector circuit D, composed of, for example a complimentary transistor circuit and according to the polarity of the output, one of the elements becomes conductive and the other becomes non-conductive in the detector circuit D. The output of the detector circuit D is put into the amplifier circuit A and the positive or negative driving current passes through the motor M to make it rotate in a normal or reverse direction.

The resistance of the variable resistor in the comparator circuit is made to change in response to the rotation of the motor M as shown with the feed back loop Fs and the comparator circuit B is brought back to a balanced condition. As shown in FIG. 9, an electronic or electromagnetic chopper circuit C is inserted between the detector circuit D and the amplifier circuit A, or in the motor circuit whereby the driving current passing through the motor is made intermittent. The period of intermittence is given with a suitable value by regulation of the chopper circuit C to eliminate the hunting phenomenon. The hunting of the motor M is caused by a mechanical inertia, a response velocity of an electric circuit, etc. and can be eliminated by the property selected speed of the motor. As a chopper cir- 3 cuit C, various pulse oscillating circuits may be used, as well as an electromagnetic relay, etc.

FIG. It) shows a modification of the fourth embodiment shown in FIG. 9. In FIG. 10, 2' is a variable resistor, the resistance value of which is made to change by 1 a motor M 3', 4' and 5' are resistors composing a bridge circuit with the variable resistor 2'. When applied to a camera, a photographing informations such as the film sensitivity, the frame number, the picture illumination may be set by these resistors. 6 and 7 are transistors composing the first stage of the complimentary detector circuit; 8 and 9 are output stage amplifier transistors; and 30 is a current source. 31 35 are respective elements of a pulse oscillating circuit for a chopper action. 31 is a variable resistor to change the time constant in the oscillating circuit; 32 is a capacitor; 33 is a double base diode for oscillation; and 34 and 35 are load resistors for generating an output pulse on both terminals X and Y respectively. The positive and negative pulses are put in the bases X and Y of the amplifier circuit, and the motor driving current passing through the transistors 8 or 9 is made intermittent. Supposing the resistor 2 has a low resistance value, the transistors 6 and 8 become conductive and 7 and 9 become non-conductive, whereby the motor M is rotated in one direction and in interlocking with this through the loop Fs, the resistance of the variable resistor 2' is made to increase. Thus, when the resistance value arrives at a certain point, the bridge circuit is balanced and the motor M is stopped. Pulses from the pulse oscillating circuit, consisting of elements 31 to 35 are superposed at the bases X and Y of the transistors 8 and 9 respectively, whereby there flows a driving current which eliminates a hunting action through the motor M when the motor is rotated by a driving current. Thus, the motor M is stopped without a hunting action and the bridge circuit becomes balanced. In the circuit of FIG. 10, the pulse signal (or a circuit turning on or of with a pulse) may be inserted in series between one of the terminals of the motor M and the current source. 36 is a main switch with two similar switching elements associated to each other.

FIG. 11 shows another modification of the fourth embodiment shown in FIG. 9, in which a multivibrator circuit is used in combination with a circuit for deriving a motor with two driving coils 37 and 38. A bridge circuit, a detector circuit and an amplifier circuit are similar to those of the example shown in FIG. 10. 39 and 40 are transistors, composing an astable multivibrator. 411 and 42 are capacitors of the oscillating circuit, and 43 and 44 are resistors. The period of oscillation of the multivibrator is made to change by the above resistors 43 and $4. The interlocking arrangement of the resistors 43 and 44 may be preferable. As the motor having two driving coils 37 and 38 is adopted, the system can be operated by a single power source 22. When the bridge circuit is in an unbalanced condition, one of two driving coils 37 and 38 is supplied with a driving current. But, this current can pass only when one of the transistors 39 and 40 of the multivibrator becomes conductive. Therefore, the current through the coils 37-or 38 is made intermittent by the period of oscillation of the multivibrator. The hunting phenomenon can be prevented by suitably selecting this period, i.e. the pulse length through the variable resistors 43 and 44. Namely, the multivibrator circuit offers a chopper effect. 

1. A servo-control system comprising a bridge circuit having an output, a detector circuit responsive to the output of said bridge circuit, a motor, a control circuit chopping the output of said detector circuit and applying it to said motor for controlling the motor on the basis of the output of the bridge circuit, feedback means coupling said motor to said bridge circuit for balancing the bridge circuit, and a pulse generator coupled to said control circuit for pulsing said control circuit and causing it to chop the output of the detector circuit.
 2. A servo-control system according to claim 1, which further comprises a start switch for the control circuit whereby the control circuit is activated by the start switch under the control of the output of said detector circuit.
 3. A servo-control system according to claim 2 in which the detector circuit is of bi-polar structure, the control circuit is an astable multivibrator and the start switch is arranged in series with said astable multivibrator.
 4. A servo-control system according to claim 2 in which the detector circuit is of bi-polar structure, the control circuit comprises a pair of pulse oscillators, each of which oscillators is controLled by the output of the bridge circuit through the detector circuit, so as to control the frequency of the output of each of the oscillators.
 5. A servo-control system according to claim 1 in which the motor comprises two coilings of opposite polarity.
 6. A servo-control system according to claim 4 in which each of the pulse oscillators is composed of a first and second direct coupled transistors of opposite polarity, and a capacitor connected between the output terminal of the second transistor and the input terminal of the first transistor.
 7. A servo-control system according to claim 6, which comprises an iris and the bridge circuit comprises a photosensitive element, whereby the system functions as an automatic camera iris control device.
 8. A servo-control system according to claim 3, in which the astable multivibrator is of a bridge connection structure, one of the outputs from the detector circuit is supplied to one of the input terminals of the multivibrator through signal inverting element, and the motor comprises a single coiling.
 9. A servo-control system according to claim 4, in which each of the pulse oscillators contains a thyristor of four terminal constructions.
 10. A servo-control system according to claim 3 in which a pair of discharge resistors of the astable multivibrator are varied in response to the output of the detector circuit so as to control the pulsive current through the motor as well as to stop the oscillation.
 11. A servo-control system according to claim 10 in which each of the discharge resistors contains a contact switch associated with an electromagnet energized by one of the outputs of the detector circuit.
 12. A servo-control system according to claim 10 in which each of the discharge resistors is a photoresistor coupled with a lamp energized to illuminate by one of the outputs of the detector circuit.
 13. A servo-control system according to claim 10 in which each of the discharge resistors is an output circuit of one of the transistors controlled by one of the outputs of the detector circuit.
 14. A servo-control system according to claim 11 in which the astable multivibrator is of bridge connection structure.
 15. A servo-control system comprising a comparator circuit, a detector circuit, a motor, a chopper which functions as a pulsive switching circuit, pulse oscillator means coupled to said chopper circuit for causing said chopper circuit to chop at given time intervals, said detector circuit being controlled by the output of said comparator circuit, said motor being controlled by the output of said comparator circuit through said detector circuit under the control of said chopper, and feedback means coupling the motor to said comparator circuit for controlling and bringing the comparator circuit to a balanced condition.
 16. A servo-control system as in claim 15, wherein said comparator circuit includes variable resistance means and a plurality of fixed resistance means and two output terminals, said detector circuit being connected electrically with said output terminal, said feedback means operatively connecting said motor to said variable resistance means, electrical source means, said chopper forming a part of a driving circuit electrically coupling said source means to said motor and chopping the driving current of the motor from said source means on the basis of the output of said detector circuit, whereby said motor is pulsively driven.
 17. A servo-control system as in claim 15, wherein said pulse oscillator is an astable multivibrator.
 18. A servo-control system as in claim 16, wherein said pulse oscillator is an astable multivibrator.
 19. A system as in claim 16, wherein the detector circuit comprises a pair of complementary semi-conductors and the chopper comprises a pair of complementary semi-conductors.
 20. A system as in claim 16, wherein the detector circuit comprises a pair of thyristors.
 21. A system as in claim 16, wherein the pulse oscillaTor is controlled by the output of the comparator circuit through the detector circuit so as to vary the frequency of the output of the oscillator for the control of the chopper.
 22. A system as in claim 16, which further comprises a delay circuit for the control of the output frequency from the pulse oscillator to shorten the rise time of the system, said control of the output frequency being effected through the current chopped by the chopper.
 23. A servo-control system as in claim 16, wherein said pulse oscillator forms an astable multivibrator with said chopper, said astable multivibrator having a pair of switching transistors one of which operates as said chopper.
 24. A system as in claim 16, wherein said pulse oscillator includes a unijunction transistor having a gate and a capacitance-resistance time constant circuit connected with the gate of said transistor.
 25. A system as in claim 16, wherein said comparator circuit includes a bridge circuit having a plurality of branches, one of said branches including said variable resistance means and others of said branches each including one of said fixed resistance means, said detector circuit comprising a pair of complementary semi-conductors having inputs, each input of said semi-conductors being connected with the output of the bridge circuit, said chopper comprising a pair of complementary switching semi-conductors, each semi-conductor being electrically coupled with said source means through said motor.
 26. A servo-control system as in claim 18, wherein said comparator circuit includes a bridge circuit having four branches, one of said branches including said variable resistance means and the other of said branches each including one of said fixed resistance means, said detector circuit comprising a pair of complementary semi-conductors having respective inputs, the input of each semi-conductor being connected with the output of said bridge circuit.
 27. A servo-control system as in claim 16, wherein said source means comprises two electrical sources, one of said electrical sources being electrically coupled with one of said switching semi-conductors through the motor and the other of which is coupled with the other of said switching semi-conductors through the motor.
 28. A servo-control system according to claim 15, in which the detector circuit comprises a pair of complementary semi-conductors and the chopper comprises a pair of complementary semi-conductors.
 29. A servo-control system according to claim 15, in which the detector circuit comprises a pair of thyristors.
 30. A servo-control system according to claim 15, in which the pulse oscillator is controlled by the output of the comparator circuit through the detector circuit so as to vary the frequency of the output of the oscillator for the control of the chopper.
 31. A servo-control system according to claim 15, which further comprises a delay circuit for the control of the output frequency from the pulse oscillator to shorten the rising time of the system, said control of the output frequency being effected through the current choppered by the chopper. 